Exhaust systems may create problematic pressure differentials in tightly sealed buildings or other enclosures. For example, hood exhaust systems in residential kitchens can remove air at relatively high rates. Such high outflow rates combined with tight seals and little or no provisions for makeup air can depressurize enclosures and create situations in which the air pressure inside the enclosures is lower than the air pressure outside. This can lead to backdrafting of combustion appliances, introducing outdoor odors into the enclosures, reducing comfort by introducing drafts, increasing radon infiltration, creating operational issues for doors, and creating moisture and mold problems. Backdrafting is particularly problematic because it can result in an accumulation of hazardous combustion products such as carbon monoxide in the enclosure.
Makeup air replaces air removed by exhaust systems. Makeup air can be introduced naturally if the enclosure has sufficient openings for air to infiltrate. Such openings can take the form of seams in construction, gaps around windows and doors, chimneys, and appliance vent pipes. Relying on naturally infiltrating makeup air has several problems, including that the openings may exist in inconvenient locations and create uncontrollable drafts or introduce undesirable odors, the amount of makeup air is difficult to quantify without empirical testing, and the untempered makeup air may add heating and cooling loads to heating, ventilation, and air-conditioning (HVAC) systems that exceed the systems' capacities. Importantly, a reasonable pressure differential between inside and outside is below three pascals (3 Pa), and it may not be possible to infiltrate an adequate amount of makeup air at a sufficient pressure to counter the outflow rate.
Where an exhaust system is capable of exhausting air in excess of four hundred cubic feet per minute (400 CFM), or where the combination of outflow rate and insufficiency of natural air infiltration results in a pressure differential of greater than two pascals (2 Pa), then a makeup air system is needed to provide makeup air. Makeup air systems may be passive or forced. Passive systems typically rely on engineered openings (as opposed to naturally occurring cracks and holes) and have some advantages but can vary in performance depending on weather conditions outside the building enclosure (e.g., barometric pressure, wind, and temperature). Forced makeup air systems may introduce outside air into the central HVAC system which filters, conditions, and distributes it through existing ductwork, or may have their own distribution ductwork. Both options have advantages and disadvantages, and determining which is best and designing the system for a particular enclosure can be a complicated process. Further, existing forced makeup air systems simply switch on and off with the exhaust fan and without regard to whether the amount of makeup air being introduced is sufficient to compensate for the amount of air being removed.
This background discussion is intended to provide information related to the present invention which is not necessarily prior art.